Such devices are used, for example, as biosensors in a large variety of applications. One particular application is the detection or monitoring of binding affinities or processes. For example, with the aid of such biosensors various assays detecting the binding of target samples to binding sites can be performed. Typically, large numbers of such assays are performed on a biosensor at spots which are arranged in a two-dimensional microarray on the surface of the biosensor. The use of microarrays provides a tool for the simultaneous detection of the binding affinities or processes of different target samples in high-throughput screenings. For detecting the affinities of target samples to bind to specific binding sites, for example, the affinity of target molecules to bind to specific capture molecules, a large number of capture molecules are immobilised on the outer surface of the biosensor at individual spots (e.g. by ink-jet spotting or photolithography). Each spot forms an individual measurement zone for a predetermined type of capture molecule. The binding of a target molecule to a specific type of capture molecule is detected and is used to provide information on the binding affinity of the target molecule with respect to the specific capture molecule.
A known technique for detecting binding affinities of target samples utilizes fluorescent labels. The fluorescent labels are capable of emitting fluorescent light upon excitation. The emitted fluorescent light has a characteristic emission spectrum which identifies the present fluorescent label at a particular spot. The identified fluorescent label indicates that the labelled target molecule has bound to the particular type of binding sites present at this spot.
A sensor for detecting labelled target samples is described in the article “Zeptosens' protein microarrays: A novel high performance microarray platform for low abundance protein analysis”, Proteomics 2002, 2, S. 383-393, Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany. The sensor described there comprises a planar waveguide arranged on a substrate. The planar waveguide has an outer surface capable of attaching a plurality of binding sites thereon. Moreover, the planar waveguide has a plurality of incoupling lines for coupling a beam of coherent light into the planar waveguide in a manner such that a beam of coherent light propagates along the planar waveguide. The coherent light propagates through the planar waveguide under total reflection with an evanescent field of the coherent light propagating along the outer surface of the planar waveguide. The depth of penetration of the evanescent field into the medium of lower refractive index at the outer surface of the planar waveguide is in the order of magnitude of a fraction of the wavelength of the coherent light propagating through the planar waveguide. The evanescent field excites the fluorescent labels of the labelled target samples bound to the binding sites arranged on the surface of the planar waveguide. Due to the very small depth of penetration of the evanescent field into the optically thinner medium at the outer surface of the planar waveguide, only the labelled samples bound to the binding sites immobilized on the outer surface of the planar waveguide are excited. The fluorescent light emitted by these labels is then detected with the aid of a CCD camera.
While it is principally possible to detect the binding affinities using fluorescent labels, this technique is disadvantageous in that the detected signal is produced by the fluorescent labels rather than by the binding partners themselves. In addition, labelling the target samples requires additional preparation steps. Moreover, labelled target samples are comparatively expensive. Another disadvantage is the falsification of the results caused by steric hindrance of the fluorescent labels at the target sample which might interfere with the binding of the target samples to the capture molecules. Further disadvantages are the falsification of the results due to photobleaching of the labels or quenching effects.